The present invention relates to processes for confining a polluted area with the aid of a gaseous jet and more specifically to a process making it possible to have a very high ratio between the pollution level of the area and the pollution level outside said area.
It is known, particularly in nuclear installations, to isolate an area containing a machine giving off large quantities of radioactive dust, by means of an air curtain, in order to be able to easily intervene on said machine through said air curtain. An example of the application of this process is illustrated in FIG. 1, in which it is possible to see within cell 1, a vertical partition 2 defining an area 4 within which is installed a machine 6 used for cutting up nuclear fuel elements. The area 4 containing the machine 6 is linked with the remainder of the cell by an opening 8. A divergent gaseous jet 12 passing out of a nozzle 10 covers the entire surface of the opening 8, thus preventing the polluted air of area 4 from entering the clean area 5 of cell 1. The air in jet 12 is taken up by a suction orifice 14 and is passed into a circuit comprising a cyclone 16 and a fan 18. Throughout the remainder of the present text the term "range of jet 12" will be used to define the distance covered by the latter from the injection slot 7 and extending up to the orifice 14 and which corresponds to the width of opening 8.
The air leaving cyclone 16 is generally discharged to the exterior of the enclosure 1. As it still contains a certain amount of radioactive dust, it must be filtered by means of an absolute filter 19 before being discharged into the atmosphere. However, quite apart from the fact that such filters are very expensive, their replacement is a long and difficult operation, due to all the protection required as a result of the high activity of the dust. Moreover, they represent active waste, which must be conditioned or processed prior to removal. It is therefore important to reduce the frequency with which replacement takes place. Thus, only part of the air leaving cyclone 16 is discharged to the outside, whilst the other part is recycled in nozzle 10. An opening 22 in wall 23 of cell 1 enables ventilation air to penetrate the cell, in order to replace the air extracted through filter 19 by fan 20.
Such a process has serious disadvantages. Thus, as can be seen in FIG. 1, the gaseous jet 12 is formed by two parts, namely a first part called the "transition zone" extends over a length 1 from slot 7 and consists of a "potential" core or "dart" 13, which is formed only by the air injected through nozzle 10 and which, viewed in section, is substantially shaped like a triangle of length 1, the latter being equal to approximately 6 times the width e of the injection slot. The velocity of the gas is the same at any point on dart 13 and is equal to the injection velocity. The second part of the gaseous jet 12 is a zone called "the full development of the jet zone" 15, in which the air injected through nozzle 10 is mixed, by suction, with the air present in cell 4 and in the clean area 5 of enclosure 1. The full development of the jet zone extends beyond the tip of the dart, specifically from the latter up to the suction orifice 14. As zone 15 has very turbulent operation conditions, it sucks in a large proportion of the dust contained in cell 4 (arrows 9). Beyond a certain concentration in jet 12, there is a risk of the dust being discharged into the clean area 5 of the enclosure by turbulent diffusion. In the most unfavourable case, the concentration of the pollution in area 5 can be roughly half its level in area 4 whereas, particularly in the nuclear industry, a minimum concentration ratio is required and this is generally below 1:100. Moreover, the dust activates the fixed installation in area 5 of cell 1 and the thus produced cumulative contamination can be very high after a certain time. In order to obviate this deficiency, attempts have been made to increase the velocity of the air blown in, but this leads to no improvement and only increases operating costs.